1. Field of the Invention
The present invention relates to fuel cells, and particularly to enhancing the flow for unit cell headers of fuel cells.
2. Description of the Related Art
Fuel cell systems are currently being developed for numerous applications, such as automobiles and stationary power plants, where they will be used to economically deliver power with significant environmental benefits.
Preferred fuel cell types include solid polymer fuel cells that comprise a solid polymer electrolyte, otherwise referred to as an ion exchange membrane, and operate at relatively low temperatures. The membrane is disposed between two electrodes, namely a cathode and an anode, forming a membrane electrode assembly (“MEA”). Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst induces the desired electrochemical reactions at the electrodes. During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The electrons pass through an external circuit, creating a flow of electricity.
The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, in addition to good sealing between fuel cell components.
A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
In a fuel cell, these plates on either side of the MEA may incorporate flow fields for the purpose of directing reactants across the surfaces of the fluid diffusion electrodes or electrode substrates. The flow fields include fluid distribution channels separated by landings. The channels provide passages for the distribution of reactant to the electrode surfaces and also for the removal of reaction products and depleted reactant streams. The landings act as mechanical supports for the fluid diffusion layers in the MEA and provide electrical contact thereto.
In the assembled stack, the aligned fluid header openings form internal manifolds or headers for the supply and exhaust of reactants to the channels in the fluid flow field plates. The fluid reactant streams are supplied to and exhausted from the headers via oxidant inlet and outlet ports and respectively, and fuel inlet and outlet ports.
In a stack, high velocity flow from the fuel cells enters the header outlet at right angles to the direction of header flow. As the high velocity flow impinges onto the main flow, it reduces the amount of cross-sectional area through which the main flow can travel in the header direction.
Furthermore, a “crescent moon” of product water has a tendency to form between the plate pinch cuts in the port area. There is a propensity for this water formation to be sucked back into the small ducts between the port and transition region by capillary action following a purge event. This water has been shown to subsequently cause blockage in these areas restricting flow to the cell.
Previously, the size of the header openings would have been adjusted in response to any cell-to-cell flow sharing problems. For example, U.S. Pat. No. 6,984,466 proposes widthwise uniformity of flow across the anodes and cathodes is improved by forming each of the header openings into a plurality of smaller, parallel flow passages. The shortcoming of this solution is that simply resizing the header flow area does not necessarily increase its utilization or reduce the wasted space caused by this effect.
Accordingly, although there have been advances in the field, it would be desirable to enhance the main flow for the unit cell headers of fuel cells.